fix(iso): auto-detect and install kernel headers at build time

- Dockerfile: linux-headers-amd64 meta-package instead of pinned ABI;
  remove DEBIAN_KERNEL_ABI build-arg (no longer needed at image build time)
- build-in-container.sh: drop --build-arg DEBIAN_KERNEL_ABI
- build.sh: apt-get update + detect ABI from apt-cache at build time;
  auto-install linux-headers-<ABI> if kernel changed since image build

Image rebuild is now needed only when changing Go version or lb tools,
not on every Debian kernel point release.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/Dockerfile

@@ -1,7 +1,6 @@
 FROM debian:12
 
 ARG GO_VERSION=1.24.0
-ARG DEBIAN_KERNEL_ABI=6.1.0-43
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -24,7 +23,7 @@ RUN apt-get update -qq && apt-get install -y \
     gcc \
     make \
     perl \
-    "linux-headers-${DEBIAN_KERNEL_ABI}-amd64" \
+    linux-headers-amd64 \
     && rm -rf /var/lib/apt/lists/*
 
 RUN arch="$(dpkg --print-architecture)" \